We start with a lattice model describing the second-order DSM. Its Bloch Hamiltonian is written as
\(H({k})=H_0({k})+H_1({k})\)  where[1]}, [2]}
\(H_0({k}) \! \! &=& \! \! t \! \sum _{j=1,2}S_j\Gamma _j-t_3 \! \left[C_3+\! \sum _{j=1,2}(1-C_j)\right]\! \Gamma _3 \ , \nonumber \\H_1({k}) \! \! &=& \! \! t_1[(C_1-C_2)\Gamma _4+S_1S_2\Gamma _5] \ . \) 
